The present invention relates generally to quadrature amplitude modulation (QAM), and more specifically to communications using a cross-constellation QAM code.
QAM signaling is often used for communications between devices. QAM signaling provides efficient use of available bandwidth by mapping symbols, generally comprising a number of bits, into phase and amplitude modulated signals.
A commonly used QAM mapping scheme for wireless communication channels uses a 64QAM Gray-labeled constellation. In a 64QAM constellation symbols of six bits, or 64 possible values, are mapped to positions representing a combination of eight possible amplitudes and eight possible phase shifts. For a Gray labeled constellation adjacent positions differ by one bit. Gray labeling provides for reduced data loss arising from corruption of information during transmission, as the most likely errors are slight pertubations in phase or amplitude. In addition, Gray labeling is often done in such a way that bits making up a symbol are separated into in-phase component bits and quadrature component bits, allowing for increased ease of demapping. A 64QAM Gray-labeled constellation thereby provides for efficient transmission of data while allowing for robust communications.
Increasing the amount of information contained in transmitted symbols provides a convenient method of increasing the data rate of a communication channel. For example, use of a 128QAM or a 256QAM constellation would allow each transmitted symbol to represent 128 values in 7 bits or 256 values in 8 bits, respectively, as opposed to the mere 64 values in 6 bits provided by a 64QAM constellation. However, use of a 256QAM constellation may require, for example, the use of 16 possible amplitudes and 16 possible phase offsets. Such a requirement may overly tax the ability of a reception system to differentiate received signals in view of various considerations such as channel noise, device capabilities and other factors. Use of a 128QAM constellation may avoid some of these difficulties while still providing an increased data rate. Unfortunately, Gray labeling of a constellation, and the increased ability to recover received data, is generally only available for constellations providing 22n values, that is square constellations.